242 GEOLOGICAL SURVEY OF TEE UNITED STATES TERRITORIES. 



weak external ligament. Again, on adjusting together match-valves, and 

 then parting their ventral margins a little so as to he able to see the inner 

 side of the hinge, the pit in the right valve is seen to he only partly filled by 

 the cartilage-process of the left, thus leaving ample space for an internal 

 cartilage, exactly as in Corbula proper. On mentioning these conclusions to 

 Mr. Conrad, he wrote back that he was satisfied that they are correct, and 

 that the group would have to rest on the oblique subspiraj character of the 

 beaks, the obliquity of the cartilage-process, and the brackish-water habits of 

 these mollusks. 



Mr. Dall, however, who adopts the group under Mr. Conrad's name 

 Anisothyris, as a subgenus under Corbula, maintains that some of the asso- 

 ciated shells are true marine types, and therefore argues that these forms for 

 which the new genus was proposed may also be marine, or" at least have 

 lived in an estuary but little diluted by the influx of the fresh water that 

 brought in the fresh-water types ; and, consequently, that they may not he 

 generically distinct from the marine typical forms of Corbula. 



That these shells are, as a whole, peculiar in the obliquity and subspiral 

 character of heaks, and their oblique cartilage-process, as well as in the pos- 

 session, in some specimens, of a rudimentary posterior lateral tooth, is not 

 denied. But, as remarked by Mr. Dall, it would hardly be possihle to draw 

 up a generic diagnosis that would separate them entirely from Corbula, and 

 at the same time include all of the species: because, in some the rudi- 

 mentary posterior lateral tooth is wanting, although at the same time the 

 specimens without this rudimentary tooth agree with the others in all other 

 respects ; while we do sometimes see obscure traces of such a tooth in true 

 marine species of Corbula. Again, the species vary in obliquity as well as in 

 the degree of the subspiral character of the beaks.* Some are also very 

 inequivalve, while others are almost equivalve. About the only distinguishing 

 characters, therefore, left, are the obliquity of the cartilage-process and the 

 general obliquity and subspiral character of the beaks. 



Mr. Woodward, of the British Museum, in the paper cited in the syno- 

 nymy, expressed the opinion that this group is not only distinguished from 

 Corbula by the forward obliquity of its beaks, hut by having the cartilage-pit 

 and tooth in the left valve, and the cartilage-process in the right, instead of 



"Corbula (Anisothyris) ledaforms, Dall, from exactly the same locality ami position as the other 

 species, has its beaks nearly erect, or but little oblique. 



